removes odors, smoke, or airborne chemical compounds from bathrooms, kitchens, or laboratories. “Ventilation and air distribution are critical with respect to the issues [chemical, biological and radiological] agents entering buildings, their movement within buildings and their subsequent removal” (Persily, 2004).

Several factors determine exposure to biological and chemical agents released indoors, including the dynamic movement of agents throughout indoor environments. The concentrations will depend on the amount of agent generated, its chemical and physical properties, and how and where it is introduced. How an agent is consequently distributed depends on many factors of the built environment, including the air ventilation system and the characteristics of the interior surfaces.

Occupant behavior also can affect the distribution of pollutants. The actual exposure and dose will depend on gender, age, metabolic activity, clothing, behavior, and susceptibility, among other demographic and personal factors. The U.S. Environmental Protection Agency (EPA) published the Exposures Factors Handbook (1997), which provides values, distributions, and ranges for many physical and human factors that are applied to quantitative risk assessment. This section discusses a few of the factors that determine indoor concentration and exposures to potential biological and chemical agents. See http://www.epa.gov/ncea/pdfs/efh for more details.

EXPOSURE

Exposure to a contaminant is defined as an event or series of events that occur when there is contact at a boundary between a human and the environment with a contaminant of a specific concentration for an interval of time (NRC, 1999). Exposure is measured in units of concentration (ppm [parts per million], mass per volume) and time. Dose is the amount of a contaminant that is deposited or absorbed in the body over a unit of time. The dose of an airborne allergen or pathogen can be further defined as the amount deposited (delivered) to a specific sites such as the upper airways.

Epidemiology, controlled-exposure studies, and environmental assessments are used to determine the concentration of a contaminant in air, food, soil, dust, and water or on surfaces as a surrogate for exposure and for dose. It is important to distinguish surrogates from actual, measured exposure or doses because, in the context of biological and chemical assessments, measurements are not likely to be available. In some cases, only the presence of a substance in the air or on surfaces will be detected. Even with concentration data, the variability in actual inhalation or deposition exposures and variations in susceptibility within a heterogeneous population make estimates of risk inherently uncertain.

Consider the following example as another potential pathway leading to B. anthracis exposure: Assume that B. anthracis has been released in a small car-



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